1. Field of the Invention
This invention relates generally to machines and procedures for printing text or graphics on printing media such as paper, transparency stock, and other glossy media; and more particularly to apparatus and methods that construct text or images from individual marks created on the printing medium, in a two-dimensional pixel array, by a pen or other marking element or head that scans across the medium.
The invention is particularly beneficial in printers that operate by the thermal-inkjet process--which discharges individual ink drops onto the printing medium. As will be seen, however, certain features of the invention are applicable to other scanning-head printing processes as well.
2. Prior Art
U.S. Pat. No. 5,065,169, of Vincent et al., introduces the importance of controlling pen-to-printing-medium distance, and flatness of the medium, in an inkjet printer. The entire disclosure of that patent is hereby incorporated by reference into this document. Vincent discloses one way of performing those functions by means of a spacer formed as a skid, roller or the like that travels with the pen.
That system performs well and is very useful--particularly in the context of a printer that has a single pen. In a multiple-pen printer, however, to facilitate simultaneous printing the pens advantageously are staggered along the direction of printing-medium advance; in such a situation a skid or roller closely associated with each of one or more trailing (downstream) pens would likely smear the ink deposited by one or more leading pens.
Under some circumstances the patented system might possibly serve even for a dual-pen printer if the skid on the trailing pen were spaced adequately behind the pen, as the skid might still be able to control the pen-to-medium distance adequately at a slightly greater distance from the pen. Due to accumulated stagger distance, this solution would be significantly less satisfactory for a four-pen printer such as is typically employed for color-plus-black inkjet printing.
Even in such cases the patented system might conceivably serve if the printing medium were limited to paper, for ink might be absorbed by the paper quickly enough to permit sliding or rolling of the spacer device over a printed area without smearing the deposited ink. In particular such a system might be rendered adequate with evaporative drying enhanced through aids such as a heater or fan, or slow throughput (printed area per unit time) to extend drying time, or combinations of these provisions.
Modern color-plus-black printers, however, are called upon to print transparencies and also to print on other glossy printing media--and to perform these feats at high speed. These plastic printing surfaces are much less absorbent than paper and typically require a heater or fan, as well as special printing modes, just to obtain adequate drying speed and throughput--without regard to stabilizing ink-drop flight distance or flattening the medium.
In fact use of a heater has become commercially important to hasten drying and has in turn introduced still other problems. In a heated print zone, changes in the temperature and humidity of a printing medium cause the medium (especially paper) to deform--both in and out of the plane of the medium. The problem addressed here is that out-of-plane deformation can cause either a decrease in print quality or collision of a leading edge of the medium with part of the mechanism--e.g., a so-called "paper crash" or "paper jam".
Failures of the printing medium to pass smoothly through the apparatus can manifest themselves in tearing or folding of the medium, or in smearing of the printed image. Whatever the form, such failures are very costly in terms of wasted material and time, and also in operator frustration; and therefore strongly affect the acceptability of a printing machine.
Hence other solutions have been sought. FIGS. 4 and 5 illustrate a representative paper-guide or hold-down-plate arrangement that has been employed in one printer available commercially from the Hewlett Packard Company as that firm's Model XL300 PaintJet.RTM..
As can be seen, the arrangement provides a single hold-down plate 121 that extends completely across and beyond the entire width of the largest size of printing medium 130' accepted by the unit--thus covering and controlling not only a relatively small or narrow sheet 130 but also a relatively large or wide sheet 130'. In the system under discussion the downstream or output edge 122 of the hold-down plate 121 is nearly tangent to the top of the drive roller 125, and spaced just slightly above the roller surface.
The plate 121 is upstream (along the direction 133 of paper advance) from a preferably heated print zone 134--which is the operating region of the nozzles 111 of one or more pens 110--or in other words along the input side of that zone 134. (To keep the diagrams simple and therefore clear, only one pen 110 is shown; but ordinarily in such systems three color-ink pens and one black-ink pen are present, and the single pen in the diagrams is to be understood as representative of all four.) A pinch roller 124 in turn is upstream from the plate, but positioned partway down around the drive roller 125, to hold the printing medium 130 in tight contact with the drive roller 125.
The drive roller 125 is about forty-five millimeters in diameter, and the pinch roller 124 about twelve. To avoid smearing ink deposited in the print zone 134, and also to avoid interference with one or more tension rollers 127 and particularly one or more mating star wheels 126, no plate is provided on the downstream--or output--side of the print zone 134.
(FIG. 6 shows what is meant by a "star wheel": the hub 45 and rollers 46 are molded together from a material commercially known as "Acetal.RTM.", which is twenty-percent Teflon.RTM.; and the sharp traction gears or "stars" are of fully hardened industrial-specification 302 stainless steel. The specific configuration illustrated is not prior art, but rather is a preferred form for use in the present invention.)
The hold-down plate 121 holds the medium 130 or 130' flat, immediately adjacent to the print zone 134; that is to say, the pen or pens 110 print close to the plate 121 but not on it. By holding the medium 130, 130' flat, the plate 121 generally deters paper jams and enhances print quality.
Through extensive observation and experiment, however, it has been found that the plate 121 does not prevent paper jams and optimize print quality consistently. Sometimes the lateral edges 135L, 135R (or 135L', 135R') of the page 130 (130') curl upward; this deformation requires raising the carriage (not shown) and pens 110, to avoid collision--which in turn lowers print quality by causing uncertainty in time of flight (as explained in the Vincent patent) and by causing spray.
Also addressed to the problems of print-medium deformation is another part of the system illustrated in FIGS. 4 and 5. The tension roller or rollers 127 and star wheels 126 disposed at the output or downstream side of the print zone 134.
The tension roller 127 and star wheel 126 are centered a distance 128 of some 41/2 centimeters from the drive-roller 125 centerline 125C. They are also about that same distance from the downstream edge 122 of the hold-down plate 121.
The tension roller 127 is typically about nineteen millimeters in diameter, and the star wheel 126 about six. The tension roller 127 and star wheel 126 constrain the medium 130 (or 130') in two ways.
First, the star wheels 126 constrain the medium 130, 130' vertically against the tension roller 127. Secondly, in the region between the two pairs of rollers 124/125, 126/127 the tension roller 127 and star wheel 126 hold the medium 130 taut and therefore relatively flat.
To accentuate this second effect, the tension roller can be overdriven. This means that the tension roller 127 and thereby the star wheel 126 are driven at a slightly greater rate than the drive roller 125, but with a clutch arrangement or the like to allow for slippage.
This part of the system too, unfortunately, is not always entirely adequate in constraining the medium enough to prevent a jam. In fact through observation and experiment it has been found that the leading edge 131 or 131' of the medium sometimes strikes one or the other star wheel 126 too high.
More specifically, the medium sometimes strikes a star wheel 126 above the point on the wheel at which that wheel can capture the edge 131, 131' and channel it properly downward against the tension roller 127. The result is a paper crash or jam--spoiling the sheet 130, 130' of printing medium, interfering with operation, and usually requiring operator intervention to clear the mechanism and reinitiate proper passage of a fresh sheet through the printer.
Printing machines of the type under discussion are also subject to a related problem. When the trailing edge 132 of the printing medium passes the pinch roller 124, the medium is no longer taut and is driven solely by the downstream tension roller 127 and star wheel 126.
With careful mechanical design, the effects of the absence of tautness as such can be rendered unimportant; but curiously the fact that the tension roller 127 has become the only driver has a significant adverse consequence. If the tension roller 127 is relatively small in diameter--as compared for example with the drive roller 125--then the relative accuracy of the printing-medium advance by the tension roller is necessarily poor.
In operation of this type of printing machine, periodically the printing-medium advance mechanism 124-127 is actuated to advance the medium stepwise--by some normal distance 41 (FIG. 7) at each step. This typically occurs between repetitions of scanning the print head 110 across the printing medium 130.
Accordingly, on the one hand, with a small tension roller, the amount of printing-medium advance cannot be controlled accurately in the end-of-page region after the drive roller can no longer engage the sheet. A result is significant mutual misalignment of successive printed swaths resulting from successive print-head scans.
The mutually misaligned swath borders appear conspicuously, making each swath stand out visually as a separate printed strip or band rather than blending smoothly into a single image. This undesirable effect accordingly is called "banding".
Banding is noticeable in large part because the positioning error accumulates or accrues over a significant distance of paper advance. That distance (in a three-pass system with a pen having ninety-six nozzles, and approximately twelve nozzles per millimeter) is the height 41 of one-third of a swath, or typically thirty-two pixel rows--equalling roughly 21/2 millimeters (one-tenth inch).
If, on the other hand, the tension roller is instead made relatively large in diameter, then the starwheel/tension-roller contact area is forced further from the print zone, diminishing control over the printing medium in that zone. What is desired is both accurate advance and good control of the medium.
The end-of-page region under consideration here has a height 140 (FIG. 7) corresponding approximately to the distance 128 (FIGS. 4 and 5)--measured along the printing-medium 130 path--between the contact areas of the two roller pairs 124/125, 126/127. As can be seen from FIG. 5, this distance substantially equals the direct center-to-center distance 128 between the drive and tension rollers 125, 127, plus roughly a quarter the circumference of the drive roller 125.
The total, based on dimensions recited earlier, is roughly nine centimeters (31/2 inches). Accordingly, in the prior-art system illustrated, the banding effect is not only significant in magnitude and therefore quite noticeable, but also extended over a distance 140 (FIG. 7) which is a rather large fraction of the height of each sheet.
Some leading-edge and trailing-edge problems of printing-medium control are sometimes addressed by inhibiting printout near the leading and trailing (top and bottom) edges of each sheet. The necessity for heating the medium in those areas is thereby obviated, reducing curl etc.
This technique can reduce the likelihood of unrestrained corners being in the print zone and so minimize the likelihood of crashes. Unfortunately, however, as will be appreciated this technique produces unacceptably large top and bottom margins.
In summary, prior systems are sometimes subject to paper crashes particularly near the leading edge of each sheet, degraded image quality due to curling and other flight-time-related errors particularly along the lateral edges over the full height of each sheet, and banding near the trailing edge. As can now be seen, important aspects of the technology which is used in the field of the invention are amenable to useful refinement.